Grid Independent
In some cases we may wish to leave buildings powered by solar power disconnected from the grid. This means that the solar cells connected to the building supply only that building (or buildings) with power. This also means that the solar cells are the only power that the connected buildings have. In order to provide power in times of little or no sunlight, batteries can be used to store the energy from the day. These batteries will be charged with the excess power generated during the day and will then slowly discharge overnight. Another option would be to have backup generators providing power using natural gas or gasoline. Grid independent applications are ideal for remote locations where the power company has yet to provide service.
If the seam is too deep beneath the surface, the company will turn to the pair of underground mining techniques. The first of these, long wall mining, makes use of a highly specialized machine to extract the coal from one whole section of the seam by loosening it from the rock and allowing it to fall on the conveyor belt which will move it to the surface. Using the second technique, room and pillar mining, the coal seam is cut into a series of rooms, making sure to shape roof-supporting pillars to avoid collapse. On a related note, once the seam is exhausted, the process known as retreat mining involves the extraction of coal from those pillars. As you might imagine, this process must be undertaken with extreme care for the safety of both workers and equipment.
Grid Connected
A city is full of building rooftops. Those rooftops are typically wasted space, which makes them the perfect place to put solar cells. Typically cities already have access to a power grid, so how can the DC solar cells be made to work with the AC power grid?
DC power can be converted to AC power by the use of a device called an inverter. An inverter will change positive electricity into negative charge. If you cycle the inverter on and off at the same frequency as the power grid, you can generate AC power that matches the grid. The "on and off" switching is done by a device called a conditioner. The conditioner also prevents too much power from making its way onto the grid at once to protect the solar cells as well as the power grid.
Buildings with solar cells can be self-sufficient and provide their own power. Typically that building does not consume all the electricity that is generated by its solar cells. So what can be done with the excess? By connecting the building to the grid it can provide power to other customers who are consuming power. This reduces the fuel consumed at the power station. It also offers the providers with a way to get power when cells are not generating power, like at night or during rain.